Insulation - foil under cement board?

I have a tongue and groove wood floor over a dirt filled crawl space
and I don't want to rip up the floor to insulate. I am going to put
cement baord down and tile the top with granite using an underfloor
heating mat since there's no heat in that part of the house.
Can I put some kinds of foil between the wood and the cement board to
keep the heat in the house? Any ideas? I have seen 2 kinds of foil one
that looks like bubble wrap and another that's like think kitchen foil.
I was thinking the bubble wrap would cause too much give and make the
tile crack?
Would the foil cause any kind of moisture problems that would rot out
the wood?
Help!

foil works by reflecting radiant heat, and i think it requires an
airspace to have any value at all. so the answer is that the foil
would have little value IMO. do you have the room to add a layer of
rigid? if you did, you could even add a layer of 2" extruded
polystyrene (dow blue or similar) with a layer of plywood over it.
expensive, yes, but then pumping heat into a crawlspace isn't exacty
cheap either. otherwise, i think you are going to regret not tearing up
the subfloor and insulating between the joists.
Mulan wrote:

2.5" is a lot of height to lose and would mean a step into and out of
the room :-( Do you think the bubble wrap style of foil would have too
much "give" under cement board? That would give me air space?
Or maybe the roll of white polystyrene that you put under wood floors
then the foil on top and then the cement board. Would that thin poly
be enough air? I SO don't want to pull up the tongue and groove.
marson wrote:

i'm really skeptical about any thin layer. you might call the company
that makes the insulation product you are considering and ask them to
tell you the R value in your situation. also, when you find out the R
value, call and ask the manufacturer of the radiant heat system if
their product works with whatever R value you come up with underneath
it. i also think anything resilient directly under the cement board is
asking for cracked tile.
so ignore my advice if you want, but you are risking cracked tiles and
a radiant heat system that is too cold to be effective...after going
through all the labor of installing tile.

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